Materials and Methods: The participants in this prospective study were seventy 14 to 50-year-old individuals. Each participant was photographed with a digital camera twice, at three different times of the day (in the morning, at noon and in the evening) and after a 6-month interval. The reproducibility of head posture was assessed by comparing the angle between the true horizontal and the ala-tragus plane. Student’s paired t-test and repeated measure analysis were used to analyze the results. To evaluate the differences between the first and second sets of photographs, Dahlberg’s coefficient (method error) was also used. 

 Results: Repeated measure analysis did not reveal any statistically significant differences in NHP orientation at different times of the day in the initial measurements (p=0.15) or after a 6-month period (p=0.56). Dahlberg’s coefficient for all the participants during the 6-month period was 3.14˚. Paired t test showed significant differences in NHP orientation only in the morning. 

Conclusion: The time of the day during which the photograph is taken does not affect the reproducibility of NHP. However, this orientation was more stable in the evening and at noon than in the morning. No differences were found between genders. In conclusion, measurements of NHP with the ala-tragus plane were more stable than measurements based on intracranial reference planes.